1. Field of the Invention
The instant invention relates to a device used to measure or produce a differential pressure between two reference points and particularly to a device which experiences static pressures on the order of 1.0.times.10.sup.4 pounds per square inch (psi) yet which is capable of measuring differential pressures as little as 0.005 psi.
2. Discussion of the Related Art
The pressure of a liquid or gas is defined as the force the fluid exerts in a direction perpendicular to a surface of unit area. A distinction is made between "absolute pressure," which is measured with respect to a total vacuum, and "gauge pressure," which is the amount by which the pressure exceeds a reference pressure such as atmospheric pressure. Therefore, gauge pressure plus atmospheric or reference pressure is equal to absolute pressure.
There are many types of gauges which are used to measure fluid pressure. One form of pressure gauge is the U-gauge or U-tube manometer. As the name suggests, the U-tube manometer consists of a U-shaped tube which contains a predetermined amount of fluid such as water, mercury or sometimes a gas. The manometer may have each arm of the tube exposed to atmospheric pressure, or one arm may be sealed in which a vacuum exists between the sealed end and the fluid. The latter type of manometer may be used to detect or measure very low or vacuum pressures.
Referring back to the first or open manometer briefly mentioned above, one arm of the tube may be in fluid communication with a pressure source. P.sub.1, and the other arm may be in fluid communication with a second pressure source, P.sub.2, (oftentimes atmospheric pressure). When each arm is subject to the same pressure, P.sub.1 equal to P.sub.2, the height of the two fluid columns are equal. If the pressures exerted upon the fluid in each arm are different, P.sub.1 unequal to P.sub.2, the height of the fluid columns will be different: the lowest mercury column being the one experiencing the greater pressure. The height difference between the two fluid levels, together with a knowledge of manometer fluid density, can provide a measure of the pressure differential between the two arms of the manometer.
To determine the level differences between the two fluid columns, the U-tubes are typically manufactured from glass or transparent plastic. This way the fluid levels could be visually measured against gradations either etched in the arm or marked on a scale adjacent each arm. One disadvantage in using a glass or plastic manometer is accuracy. The height of the fluid column can only be measured to within a limit defined by the scale of the gradations. Secondly, the working fluid within the manometer tubing usually has a miniscus at the top of each column. The miniscus adds to the problem of accurately determining weight because the height of the column may not be interpreted consistently by different observers. Another disadvantage of a glass or plastic manometer is the range of static pressures at which it may be safely used. Static pressures on the order of 500 pounds per square inch (psi) may cause the tube to rupture.
In situations where the static pressure may exceed that permissible for a glass or plastic manometer, high-pressure vessels or tubing may be used. The vessels or tubes are commonly manufactured from steel or other high strength opaque materials where the height of the enclosed fluid columns cannot be visually determined. In the past, mechanical devices such as floats have been attached to dials or gauges. A major disadvantage to float-type detectors in each column is that the volumn of each float changes according to the static pressure in each column. This may lead to serious inaccuracies output at each gauge. Another disadvantage is that moving parts require maintenance.
One manometer consists of several upright tubes, each partially filled by a heavy liquid, in this case, brine. The remaining volumn of each column is filed with a gas. The tubes are in fluid communication with each other to exchange quantities of brine or gas. A diaphragm closing the bottom of each tube contains an acoustic transducer. Because the diaphragm is necessarily thin, a housing surrounding the diaphragm and transducer must be pressurized to prevent the diaphragm from sagging or rupturing under the pressure of the brine. As the pressures change in each tube, the height of the respective brine may be determined by measuring the two-way travel time of an acoustic pulse sent by the transducer and reflected from the top of the brine column back to the transducer. Periodically, the two-way travel time of the acoustic pulse to a target at a known distance is measured to provide a calibration for the manometer system. Herein lies one of the major disadvantages to the above system. The calibration targets used to determine the propagation velocity of the acoustic pulse through the brine are located near the top of each tube, well into the volume occupied by the gas. In order to calibrate the system, the gas must be displaced and the volume of brine increased in order to immerse the calibration targets. The two-way travel time is then measured between the transducer and calibration target. Once calibrated, the brine level must be lowered in order not to reflect a signal from both the top of the brine and the calibration target. The requirement to alter the system for the purpose of calibrating the acoustic pulse is inefficient thereby resulting in longer operational times and increased costs. An additional disadvantage is the initial cost of the complex plumbing used to transmit the different fluids between the different tubes.